U.S. Pat. No. 4,318,400 referred to above and U.S. Pat. No. 4,386,929 to Peery et al. entitled, "Elastomeric Bladder Assembly" disclose an infusor, a device for delivering a predetermined quantity of a drug or other medicament to a patient in a pre-selected time period at a substantially constant fluid flow rate. The infusor includes a bladder which contains the liquid to be infused. The bladder is pressurized by the liquid to be infused into the patient. Such an infusor is sold by Travenol Laboratories of Deerfield, Ill., Product Code No. 2C1070. That infusor is intended for delivery of medical substances to human patients.
As disclosed in U.S. Pat. No. 4,318,400, the infusor includes a housing, a plug fixed in one end of the housing and having an aperture that extends through the plug, and a tubular elastomeric bladder in the housing for receiving the liquid under pressure. One end of the bladder is sealingly attached to the plug, with the lumen of the bladder communicating with the aperture of the plug. The disclosed infusor further includes a conduit connected to the plug aperture, the conduit and the aperture together defining a dispensing passageway for transporting the liquid from the bladder to the infusion site of the patient. A flow regulator is disposed somewhere in the dispensing passageway for permitting the liquid to flow from the bladder, through the dispensing passageway at a predetermined rate. The flow restrictor comprises a capillary element which is illustrated within the housing.
The infusor is designed to deliver the liquid medicament at a predetermined flow rate. However, with the principal flow restrictor comprising a capillary element, the extent to which the restrictor limits the maximum fluid flow rate out of the infusor is determined by the length and cross-sectional area of the capillary element itself.
If the capillary element is to be maintained within the housing, it becomes difficult to substantially lengthen the capillary element without requiring a re-design of the housing. To avoid such a re-design, the internal diameter of the capillary element may be varied; however, with the small internal diameters utilized in the flow restrictor, such as a capillary of about 0.0016 inch in diameter, it becomes difficult to consistently manufacture capillary elements having the required precise internal diameter. It should be noted that variances in diameter can average out over the length of the capillary element but as noted above, the length of the element is limited when it is disposed within the housing, thereby limiting the extent to which the effect of variances along the length of the capillary element can be minimized. The length of the capillary and flow restrictor in Product Code No. 2C1070 is about 0.316 inch.
Furthermore, although the infusor sold under Product Code No. 2C1070 is designed to deliver a relatively small volume of fluid over a relatively long period of time, such as about 48 ml in twenty-four hours, even slower flow rates are desirable in certain circumstances. As an example only it is sometimes desirable to deliver 36 ml, 48 ml or 60 ml of a medical liquid in three, four or five days, respectively.
With such extremely low flow rates, flow of fluid in the conduit downstream of the flow restrictor is extremely slow, even with a small tubing lumen. Thus, the time required to prime the infusor, i.e., the time to purge the aperture and conduit of air by filling same with liquid, is about fifteen to twenty minutes for example with the infusor sold under Product Code No. 2C1070.
With longer flow restrictors required for slower flow rates the priming time will grow considerably. For example, to delivery the same volume of liquid in four days instead of one day, the flow restrictor will be about 4 times longer, with the same capillary diameter. This would make the priming time about 4 times greater, to about 60 to 80 minutes. It would be desirable to provide an infusor having slower flow rates without increasing the infusor priming time.
The infusor sold under Product Code No. 2C1070 includes O-rings at both ends of the flow restrictor to prevent fluid flow between the housing for the flow restrictor and the outside of the flow restrictor. Because the flow restrictor is between two other elements in the housing the restrictor needs to be sealed with O-rings at both ends. While providing a highly effective seal, it would be desirable to reduce the number of required sealing elements, or O-rings. Also, because the O-rings are disposed at the ends of the flow restrictor, any housing re-design, referred to above regarding a longer capillary element, would necessarily require re-designing the placement of the O-rings in the housing.
It has also been found that the temperature of the liquid to be delivered to the patient by the infusor has a significant effect on the fluid flow rate. This is because the temperature of the liquid affects its viscosity. It would be desirable to provide for a constant liquid temperature in order to provide a constant flow rate. It would also be desirable to know the temperature of the liquid during design of the infusor so that the preselected fluid flow rate could be more easily achieved.